1. Field of the Invention
This invention relates generally to a method for producing alkali metal gold cyanide and, more particularly, to a method for producing alkali metal gold cyanide starting from a mixture of finely divided gold metal particles, hydrogen peroxide, and alkali metal cyanide.
2. Background of the Prior Art
Potassium gold cyanide has become a very popular chemical compound in the semiconductor industry for use in electroplating gold to be used as contacts on semiconductor devices. As a result of this great importance of the potassium gold cyanide material, it soon became very evident that improved processing techniques were needed to handle the gold deposited material that was not utilized in the fabrication of the gold contacts for the semiconductor devices. In carrying out the electroplating process for depositing gold onto the semiconductor devices, it often occurred that various other parts were coated with gold as it was difficult, if not impossible, to selectively control the electroplating process to only deposit gold onto the desired portions of the semiconductor devices. The natural result of this additional or excess electroplating was that gold often was deposited onto objects such as holders for the semiconductor products or other similar objects that were an integral part of the electroplating process. This was recognized by the workers skilled in the electroplating arts and various techniques were tried to develop methods for salvaging the gold deposited on undesired substrates.
Many attempts at recycling the excess gold deposited onto undesired substrates utilized electrolysis which is an inherently slow process that moreover requires the preprocessing of gold into anodes before being incorporated into electrolytic potassium gold cyanide producing cells. In order to obtain cyanide-free product, fractional crystallization was then employed as the purification process. Due to the physical properties of both cyanide and potassium gold chloride, fractional crystallization is an inefficient and therefor costly method to separate the two substances. Although the resultant final product derived from the fractional crystallization process is substantially cyanide-free, its purity, mainly with respect to silver contamination of the starting material is relatively high and is not completely removed by the fractional crystallization process. The reason for this is that the potassium gold cyanide does not undergo any purification in the fractional crystallization process and, thus, this prior art method was generally found to be unsatisfactory for the more sophisticated semiconductor manufacturer's use of high purity potassium gold cyanide materials.
Various attempts have been made to chemically produce potassium gold cyanide; however, the people skilled in the art generally considered this approach to be much more complex than the electrolysis method described above. For example, U.S. Pat. No. 3,112,174 teaches that the preparation of this compound from gold powder and potassium cyanide in the presence of an oxidizing agent such as air or a peroxide is slow and difficult to control on a small scale. Additionally, the price or cost per ounce together with the pre-refining charges to convert gold to potassium gold cyanide using the prior art techniques was extremely high.
As a result, a need existed to find a method for overcoming all of the above disadvantages associated with the prior art methods for producing potassium gold cyanide. Additionally, the new method would have to produce potassium gold cyanide without the previous preprocessing step of producing the gold into a foil, but would utilize finely divided gold metal particles in the form of gold sand which is the usual final product of a gold reclaiming and refining process that exists in semiconductor manufacturing gold salvage operations.